A review on mechanisms and recent developments in p-n heterojunctions of 2D materials for gas sensing applications

Tailoring the selectivity of ultralow-power heterojunction gas sensors by noble metal nanoparticle functionalization

“…At the working temperatures OPTs of 250C, 300C and 350C, the sample already has p-type conductivity and is therefore selective to n-butanol, where the responses are 54%, 200%, and 163%, respectively. This phenomenon is dictated by the composition of the sample, which change the surface activity and reaction products [7,22,23] by band bending and formation of electron depletion or space charge layer in the conduction band, so the conductivity of the active materials either decreases or increases [76]. The lower operating temperature may not influence the depletion layer of electrons in TiO2, and the CuO/Cu2O layer has a higher resistance, the current flows through the TiO2 layer which is an n-type semiconductor, but at a higher operating temperature the depletion layer of electrons in TiO2 and the accumulation layer of holes in CuO/Cu2O are also influenced, thus leading to the CuO/Cu2O layer predominating in the sensing mechanism leading to the increase of the heterojunction resistance to the application of the target gas [77].…”

Emerging MXene–Polymer Hybrid Nanocomposites for High-Performance Ammonia Sensing and Monitoring

“…Currently, 2D sensing nanomaterials owing to their larger specific surface area, tunable chemistries, and their fabrication techniques, have been extensively studied in terms of three S’s (sensitivity, selectivity, and stability) and five essential R’s (room temperature operation, range of detection, repeatability, response and recovery, and reproducibility) [ 11 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. A 2D material (such as graphene and its derivatives, 2D organic frameworks, and polymers, black phosphorus, borophene, metal dichalcogenides, phosphorene, molybdenum disulfide, metal carbides, or nitrides (MXenes)) is generally stated to an atomically thin layer or few layers of flakes with thickness varying from 1–5 nm [ 14 , 21 ].…”

“…A 2D material (such as graphene and its derivatives, 2D organic frameworks, and polymers, black phosphorus, borophene, metal dichalcogenides, phosphorene, molybdenum disulfide, metal carbides, or nitrides (MXenes)) is generally stated to an atomically thin layer or few layers of flakes with thickness varying from 1–5 nm [ 14 , 21 ]. Although various conventional 2D materials, including graphene or metal dichalcogenides, have shown promising NH 3 sensing properties due to their high specific surface area and tunable chemistries, they are lacking due to complex synthesis techniques, difficult surface functionalization, and poor selectivity [ 18 , 19 , 20 ]. MXenes are one of the recent and most popular NH 3 sensing materials owing to their excellent hydrophilicity, excellent conductivity, large surface to volume ratio, biocompatibility, high mechanical strength, and enriched surface chemistry (due to the presence of surface terminated groups such as –OH, –O or –F) [ 22 , 23 , 24 ].…”

“…MXene–polymer nanocomposites have recently aroused inordinate interest in NH 3 sensing applications. However, there are reports on nanocomposites of polymers with other 2D materials such as graphene and its derivatives, metal dichalcogenides and molybdenum disulfide [ 11 , 14 , 17 , 18 , 19 , 20 ]. However, the fabrication and functionalization of such nanocomposites is tedious, which limits their application in ammonia monitoring [ 11 , 14 , 17 , 18 , 19 , 20 ].…”

“…However, there are reports on nanocomposites of polymers with other 2D materials such as graphene and its derivatives, metal dichalcogenides and molybdenum disulfide [ 11 , 14 , 17 , 18 , 19 , 20 ]. However, the fabrication and functionalization of such nanocomposites is tedious, which limits their application in ammonia monitoring [ 11 , 14 , 17 , 18 , 19 , 20 ]. As well, the presence of surface functional groups over MXene makes them more prone to polymers for formation of enhanced hybrids through various multi-interactions such as hydrogen bonding, van der Waals forces, covalent bonding or electrostatic interactions [ 35 , 36 ].…”